Composite materials are known, wherein a fiber reinforcing material is impregnated with a resin which is cured to give a strong, lightweight material. Such materials are particularly suitable for structures where the low weight to strength ratio may be exploited, for example in aircraft structures.
Conventionally, some lightweight structures are made of lightweight metal alloys and in some of these structures one or more sheets of the structure are perforate to provide sound attenuation. Such perforate sheets are found, for example, on the face of blocker doors in jet engines. These perforate metal alloy sheets may be made by known techniques of casting and drilling or punching, and the like. Hitherto, the use of resin-impregnated fiber-reinforced composite materials would not have been thought applicable to the manufacture of such perforate sheets because, drilling or punching fiber-reinforced composite materials would have been expected to break the continuity of the fibers with loss of strength. Furthermore, such additional processing is time consuming.
If the perforations in the reinforcing fiber material are formed before it is impregnated and cured, by means of needles in a mold, the fibers of the reinforcing fiber material may be parted by the needles rather than being broken, thereby maintaining the continuity of the fibers. This results in a composite material with superior qualities.
A problem often associated with the manufacture of composite materials with intricate shapes, such as, for example, numerous perforations, is that the material is difficult to remove from the mold. A number of attempts have been made to solve this problem. For example, United Kingdom Patent Application No. GB2090183A relates to a method of manufacturing a perforate composite material wherein a former is melted to release the composite from the mold. Such processes may be very time consuming.
GB 2025302A describes a process in which a fibrous sheet is impregnated with liquid resin, and the sheet is then pierced with hot pins so that the resin around the pins is partially cured. The pins are then removed, and the sheet is fully cured by application of further heat.